Comparison of site classification from VS30, VS10, and HVSR in Italy

The aim of this work is to verify if (1) the average of shear-wave velocity from the surface to 30 m depth (VS30) is a good proxy of site amplification in a country with complex geology like Italy, (2) the grouping of VS30 in different soil classes and relevant spectra in the Italian seismic codes is adequate, and (3) shallow shear-wave profiles (VS10) could be a more economical tool for site classification. To answer the first two questions, we examined 40 estimates of VS30 derived from microzonation projects we performed in Italy (27 obtained with downhole measurements, plus 13 velocity profiles obtained with surface techniques). In all the sites, we installed seismic instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios (HVSR). The comparison between HVSR and VS30 showed in about one-third of the sites that VS30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile. The reason VS30 does not provide satisfactory estimates in Italy is linked to peculiar geological settings that are widespread in the country. We then compared the observed amplifications from earthquake HVSR data with the ones provided by the Italian seismic code, noting a substantial underestimation by the code, a somewhat unsettling situation because HVSR is usually considered to be a lower bound for amplification estimated with other techniques. Finally, we studied 45 VS downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available. On this data set, we noticed that VS10 could predict site classification with the same performances of VS30. We consider alternative soil classification schemes that include soil frequency besides the velocity profile. In this two-parameter approach, VS10 could be substituted for VS30.